The present invention relates to the field of semiconductor sensors, and in particular to a semiconductor sensor with a field-effect transistor.
U.S. Pat. No. 5,911,873 describes the linking of field-effect transistors to measure ion concentrations in liquids and gases. U.S. Pat. No. 4,411,741 describes the determination of gas concentrations by measuring work function differences on gas-sensitive layers. German patent application DE 31 23 403 A1 describes a semiconductor sensor that can be switched by a control device from a measuring phase to an idle phase.
Also known are sensors in which one drain terminal and one source island are generated by counterdoping within one semiconductor substrate, and an insulating layer is grown or deposited on the substrate between the source terminal and the drain terminal. These sensors are also called transducers. Depending on the application, the ion-sensitive layer is applied, or a gas-sensitive layer is located at a predetermined distance, between the source terminal and the drain terminal. The latter types are often referred to as suspended gate FETs (SGFETs).
Another application using semiconductor sensors involves an electrode on the insulator that is capacitively controlled by a gas-sensitive gate incorporated at a predetermined distance and that is connected to the gate of a sensing transistor. In regard to the field-effect transistors used, these are so capacitively controlled field-effect transistors (CC-FET). German patent application DE 43 33 875 C2 describes such CC-FETs.
In semiconductor sensors using field-effect transistors, the change in the charge or work function caused by the ion to be detected is detected by a drain-source current change in the field-effect transistor that forms the sensing transistor. In the case of the SGFET and CC-FET, one specific advantage is the fact that the transducer and sensing layer can be processed independently of one other.
CC-FETs are also known in which a control electrode is incorporated under the floating electrode to affect the floating electrode capacitance, adjust the operating point, as required, and compensate for fabrication-related fluctuations in the operating characteristic of the field-effect transistor. A disadvantage of these systems is that the potential of the floating electrode is capacitively raised unintentionally. What always occurs is that this electrode is returned by non-controllable surface conductors to the potential of the environment, generally defined by a guard ring, and as a result, the field-effect transistor drifts in terms of its operating point. In other designs as well, a large fraction of the drift is caused by application of potentials to the source terminal, drain terminal, and substrate, the potentials being capacitively transmitted via the gate of the sensing transistor to the sensor electrode.
There is a need for a semiconductor sensor with improved operating point and reduced drift, especially when the semiconductor is turned on.